1. Field of the Invention
This invention relates to a brushless motor, and more particularly to a brushless motor which can start itself with a single position detecting element.
2. Description of the Prior Art
A 2-pole field magnet, 3-stator pole, 3-coil, 3-phase small dc brushless motor as shown in FIG. 1 is already known as an inexpensive, efficient brushless motor. Brushless motors of this type are mostly used for commutator motors because they are more inexpensive and efficient than 2-phase, 4-coil small dc motors. The structure of such 2-pole, 3-coil, 3-phase small dc motors will be first described with reference to FIGS. 1 to 3. The brushless motor generally denoted at 1 includes a stator core 2 as seen in FIG. 2 which constitutes a stationary section of the motor 1, and a 2-pole field magnet 3 as seen in FIG. 3 which is mounted for rotation around the stator core 2. Thus, the brushless motor 1 includes no brush or commutator.
The stator core 2 is securely mounted on a fixed base 4 and has 3 substantially T-shaped stator poles 5 as shown in FIG. 2. The 3 stator poles 5 are connected in a circumferentially equidistantly spaced relationship to a central annular portion of the stator core 2 and extend radially outwardly therefrom. The central annular portion of the stator core 2 has a center bore 6 formed therein for receiving a rotary shaft therein. A coil 7 is wound on each of the T-shaped stator poles 5 of the stator core 2. The stator poles 5 each have a radial or stem portion 8 on which a driving coil 7 is wound and an outer circumferential portion 9 which extends in opposite circumferential directions from an outer end of the stator pole 5. The circumferential portions 9 of the stator poles 5 have an angular width smaller than 180 degrees, for example, an angular width of 90 degrees. In this arrangement, antitorque appears and hence the efficiency is low, but considerable turning torque can be obtained.
A pair of bearings 10, 11 are secured in the center bore 6 of the stator core 2 by means of a bearing support member 12. A rotary shaft 13 is supported for rotation by the bearings 10, 11 and has a cup-shaped rotor yoke 14 secured thereto. The annular 2-pole field magnet 3 is securely mounted on an inner circumferential face of the cup-shaped rotor yoke 14 in an opposing relationship to the stator core 2 and for rotation around the stator core 2. The field magnet 3 is magnetized to form a pair of N (north) and S (south) magnetic poles which have an angular width of 180 degrees as illustratively shown in FIG. 3. 2 or 3 position detecting elements 15 such as Hall effect elements or Hall ICs (integrated circuits) are located at suitable positions on the outer periphery of the stator core 2 for detecting an N or S pole of the field magnet 3 in order that the driving coils 7 may be energized successively by an electric current flow in an appropriate direction to rotate the rotor in a predetermined direction. A printed circuit board 16 is securely mounted on the base 4.
The small dc brushless motor 1 having such a construction as described above has a drawback that a high cogging torque appears and will prevent smooth rotation of the rotor when the rotor is rotated from a stable position to another stable position passing an intermediate unstable position by energization of the coils. Accordingly, during rotation of the brushless motor 1, disagreeable high noises are normally produced. Therefore, the brushless motor 1 is not suitable for a brushless fan motor for use with a business machine or an automobile in which it must be rotated with minimum noise, and great efforts have been made to minimize or eliminate disagreeable turning noises of brushless motors. However, this is very difficult to accomplish due to properties of brushless motors as rotary motors. Thus, a brushless motor as shown in FIG. 4 has been proposed to eliminate this problem. The motor shown includes a stator core having a double number of, that is, 6, stator poles 17 with coils 7 individually wound thereon. However, this also has the drawback that the efficiency is not improved significantly while the cost rises. The dc brushless motors of the constructions described above have the further drawback that they cannot be produced at a low cost because they necessitate 2 or 3 (sometimes, 4 or more) expensive position-detecting elements and hence soldering operations must be done individually for such position-detecting elements.
Particularly in small motors, it is necessary to raise the efficiency without increasing the cost or else to reduce the price with the efficiency maintained, and hence the price cannot be raised so high. Accordingly, in such small rotary motors which include a small number of components in construction and are simple in principle, a significant difference or merit in efficiency may be achieved from a little improvement thereof. Therefore, various inventions and proposals have been made so far, and hence a large number of applications for patents have been made.
Thus, a 4-pole, 3-stator pole, 3-coil, 3-phase brushless motor wherein grooves are formed on stator poles to reduce a cogging, to attain smooth rotation of the motor, is proposed in a Japanese laid-open patent No. 54-57608. This brushless motor, however, has a drawback that it is very expensive even though it is advantageous in improved performance. A 2-pole, 3-stator pole, 3-coil brushless motor wherein grooves are formed on stator poles is also disclosed in another Japanese laid-open patent No. 53-147216. However, this motor also has a drawback that it is expensive. It is another drawback of the motor that the efficiency cannot be improved very much due to generation of opposing torque.
3-stator pole, 3-coil, 3-phase brushless motors are considered superior to 2-phase brushless motors because a higher turning torque can be obtained with a higher efficiency. However, because they produce disagreeable high turning noises during rotation at high speed, increasing interest has been taken in 4-stator pole, 4-coil, 2-phase small dc brushless motors with smooth rotation. These are more suitable for applications which require minimized noise, such as fan motors, but are less efficient. Further, such 4-stator pole, 4-coil, 2-phase small dc brushless motors also have a drawback in that they require at least two position detecting elements and are therefore expensive.
A typical brushless motor which is designed to require only one position-detecting element in order to eliminate the drawbacks described above is a 4-stator pole, 4-coil, 2-phase small dc brushless motor. Now, a brushless motor of the type just mentioned will be described with reference to FIG. 5.
The 2-phase small dc brushless motor generally denoted at 18 is constituted as an outer rotor motor and includes a field magnet 19 having 4 driving alternate N and S magnetic poles and securely mounted on a rotor yoke 20 for rotation around a stator core 21. The stator core 21 has 4 T-shaped stator poles 22 formed in a circumferentially equidistantly spaced relationship thereon, and a driving coil 23 is wound on each of the T-shaped stator poles 22. The driving coils 23 are wound in bifilar windings such that two opposing ones of the armature coils 12 for a first phase may be energized with a polarity opposite to the polarity of the other opposing armature coils 12 for a second phase. The brushless motor 18 further includes a Hall IC 24 as a position-detecting element.
The brushless motor 18 of FIG. 5 is characterized in that the stator poles 22 of the stator core 21 are each sloped or slanted with respect to the rotor such that the dimension of the air gap gradually increases toward a clockwise direction in order to allow self-starting of the motor with the single position-detecting element 24. The brushless motor is thus relatively simple in construction, but it has a drawback that, because the dimension of the air gap is relatively large, the efficiency of the motor is low.
Another typical brushless motor is disclosed in U.S. Pat. No. 3,299,635 and is shown in FIG. 6. The single-phase brushless motor generally denoted at 30 in FIG. 6 is constituted as an inner rotor motor and includes a single detecting element and a stator core having auxiliary stator poles. A stator armature core 25 has 4 radially extending main stator poles 26 formed in a circumferentially equidistantly spaced relationship, 4 driving coils 28 wound on the main stator poles 26, and 4 smaller auxiliary stator poles 27 formed between the main stator poles 26. A field magnet 29 is magnetized to have a pair of opposing N pole zones, S pole zones and O pole (non-magnetized) zones successively formed thereon. Such a 2-phase small dc brushless motor 30 as shown in FIG. 6 is surely useful because it can start itself with a single position-detecting element, but due to the presence of non-magnetized zones, it has drawbacks that torque ripples are so high that smooth rotation cannot be attained and that it is complicated in construction, expensive and inefficient.
A 2-phase small dc brushless motor which eliminates such drawbacks of the motors of FIGS. 5 and 6 as described above is shown in FIGS. 7(a) and 7(b). The brushless motor shown in FIGS. 7(a) and 7(b) is constituted as an outer rotor motor wherein a field magnet 31 as shown in FIG. 7(a) rotates around a stator core 32 as shown in FIG. 7(b).
Referring to FIG. 7(a), the field magnet 31 has 4 driving altnerate N and S magnetic pole zones 31a formed by magnetization in a circumferentially equidistantly spaced relationship from each other and 8 auxiliary alternate N and S magnetic pole zones 31b also formed by magnetization in a circumferentially equidistantly spaced relationship from each other. Thus, the ratio a:b in width between the driving magnetic pole zones 31a and the auxiliary magnetic pole zones 31b is 2:1 with each of the 4 driving magnetic pole zones 31a overlapping with two of the 8 auxiliary magnetic pole zones 31b.
Referring now to FIG. 7(b), the stator core 32 has 4 main stator poles 33 and 4 auxiliary stator poles 34 located between the stator poles 33 as in the brushless motor 30 shown in FIG. 6 (while there is a difference in that the motor 30 of FIG. 6 is constituted as an inner rotor motor and the motor of FIGS. 7(a) and 7(b) is constituted as an outer rotor motor). Armature coils 35 are wound only on the main stator poles 33 and are connected into two phase windings such that each two opposing ones thereof, which are located at symmetrical positions spaced by an angle of 180 degrees relative to the center of the motor, are connected in series so as to provide the same polarity to the associated main stator poles 33.
A Hall effect element is provided at a location not shown for detecting the position of the rotor in order that the field current of the driving coils 35 is switched by means of a transistor in response to a relative position of the field magnet 31 to the stator core 32 to obtain a torque in a predetermined fixed direction.
The brushless motor having such a construction as described above thus eliminates dead points by the combination of the composite field magnet 31 and the auxiliary stator poles 34 of the stator core 32 to allow self-starting of the motor with a single position-detecting element (Hall effect element). Meanwhile, the torque is generated by the composition of the 4 driving magnetic pole zones 31a and the 8 auxiliary magnetic pole zones 31b.
The brushless motor having such a construction as described above necessitates only one position-detecting element and has a reduced number of circuit components and is smaller in size if it is driven in half-waves. Thus, the single-phase brushless motor of the type is effective in practical use.
However, the brushless motor which is known as such a useful motor has a drawback that it is complicated in construction and difficult to produce and besides it is expensive because of the presence of the auxiliary stator poles and because a composite field magnet must be involved. Further, due to the presence of the auxiliary stator poles, it is difficult to wind a driving coil on a main pole 33 and hence it cannot be readily mass-produced. In addition, the efficiency is low due to antitorque which is caused by the main stator poles 33 which have an angular width smaller than 90 degrees while the N and S magnetic pole zones are magnetized with an angular width of 90 degrees as the brushless motor has 4 driving magnetic poles 31a for generating a main torque (however, the efficiency is naturally higher than that of conventional brushless motors).
Anyway, conventional single-phase energized brushless motors (while such motors are sometimes called 2-phase motors because they have an arrangement of driving coils for two phases, it is accurate to call them single-phase motors because they are energized for a single phase in principle) which are designed to allow self-starting with a single position-detecting element generally have a drawback that the structure is complicated and hence their production cost is high.